1. Field of Invention
This invention relates to semiconductor light detectors and to operating such devices to detect far infrared light. In particular, it relates to operating p-i-n diodes and multi-layered semiconductor devices as far infrared detectors. The invention also relates to semiconductor devices constructed to operate efficiently as far infrared detectors at selected wavelengths.
2. Background Information
The p-i-n diode is widely used as a photodetector. It comprises a layer of n-type material and a layer of p-type material separated by an undoped layer of intrinsic semiconductor material. The p-i-n diode is normally operated in a reverse bias mode. That is, the positive pole of a bias voltage is applied to the n-layer and the negative pole to the p-layer. When operated in this mode, photons excite electrons in the valence band of the layer of intrinsic material to the conduction band through which they flow into the n-layer where they are withdrawn by the positive bias to produce a current in an external circuit. At the same time, holes in the conduction band are excited by photons to the valence band through which they flow into the p-layer to add to the current. Measurement of this current provides an indication of the intensity of the light impinging on the p-i-n diode. This transition from the valence band to the conduction band in the intrinsic layer requires a sizable amount of energy. For instance, in silicon, the usual base material from which p-i-n diodes are fabricated, the required energy is about 1.1 ev. This amount of energy must be available in the photons of the light to be detected. Thus, p-i-n diodes as conventionally operated can only detect light in the ultraviolet, visible and near infrared ranges where the photons have appropriate energies to excite electrons from the valence band to the conduction band in the intrinsic layer. They can not detect far infrared light where the photons have energies of about 45 mev or less. Normally, the p-i-n diode is operated at room temperature. This does not produce unacceptable noise levels because the reverse bias keeps thermionic currents to very low levels. Materials other than silicon have been used in p-i-n diodes to detect longer wavelength light. For instance, germanium p-i-n diodes require only about 0.7 ev for transitions between the valence and conduction bands. These devices are also conventionally operated with reverse bias.
Efforts have been made to extend the range of germanium photoconductive detectors into the far infrared region by applying mechanical stress to the devices. Detection of photons having an energy of about six mev has been reported using this technique.
Another type of device used to detect far infrared light is the blocked impurity band detector. These devices utilize an infrared (IR) active layer which is doped to produce a gap between Hubbard bands. This gap is on the order of the energy level of photons in the longer wavelength end of the far infrared range. A thin blocking layer of intrinsic semiconductor material is provided between the IR active layer and a contact layer of the same type semiconductor material as the IR active layer. The device is operated with reverse bias. Photons of far infrared light excite electrons from the lower Hubbard band across the gap to the upper Hubbard band which is contiguous with the conduction band of the IR active layer. The electrons excited to the upper Hubbard band thus flow through the conduction band to the contact layer. Such devices are operated at a low temperature where thermal generation of free charge carriers is negligible. An example of such a blocked impurity band detector is disclosed in U.S. Pat. No. 4,568,960.
Recently, work has been done with detectors made of a plurality of alternating layers of doped and undoped semiconductor materials. Such devices using n-type material for the doped layers are referred to as n-i-n-i structures while those with p-type material are called p-i-p-i structures. The n-i-n-i structures have been used to detect visible light where the photons have sufficient energy to excite electrons from the valence band to the conduction as in the case of conventional operation of p-i-n diodes.
There remains a need for a simple low cost technique for detecting far infrared light.
There is an associated need for being able to detect far infrared light using currently available semiconductor devices such as p-i-n diodes.
There is a further need to be able to detect far infrared light without having to apply mechanical stress to the detector.
There is also a need for being able to easily select the wavelengths of far infrared light to be detected.
There is an additional need to increase the quantum efficiency of far infrared light detection.